1. Field of the Invention:
This invention relates in general to a packoff for sealing between a casing hanger and a wellhead housing, and in particular to a packoff having tapered sealing surfaces.
2. Description of the Prior Art:
In a subsea well, a casing hanger will be secured to a string of casing and lowered into a wellhead housing. The wellhead housing will be located on the sea floor. There will be an annulus or clearance between the outer wall of the casing hanger and the wellhead housing bore wall. After the casing has been cemented, a packoff will be installed in this annular clearance to seal the annulus surrounding the casing
A wide variety of designs for packoffs have been used and have been shown in patents. Some use elastomeric seals. Others use a combination of metal and elastomeric. Other types use metal only. One type of metal-to-metal seal has a seal member with annular inner and outer legs or walls separated by a slot. The outer leg seals against the wellhead housing. The inner leg seals against the casing hanger. The energizing member will move downward in the slot to expand the legs radially to accomplish the seal.
In some types of casing hanger seals, the wellhead bore wall and the casing hanger outer wall will be cylindrical and concentric The packoff will land on a shoulder formed on the exterior of the casing hanger below the sealing areas. In another type, the outer wall of the casing hanger in the sealing area will be slightly conical The sealing surface will taper from a larger diameter at the base to a smaller diameter at the top. This results in an annular clearance that decreases in radial width in a downward direction. In these types, the packoff will wedge into the annular clearance, rather than landing on an upward facing shoulder.